This invention relates to a method and an apparatus for measuring dissolved gas concentrations, and more particularly to a method and an apparatus for simultaneously measuring a dissolved oxygen concentration and dissolved hydrogen concentration of core water at a high temperature and a high pressure in light water or heavy water-moderated nuclear reactors.
In the present invention, dissolved gas concentrations of core water are measured on a polarographic principle. Measurement of dissolved oxygen concentration is disclosed in U.S. Pat. No. 3,328,277, U.S. Pat. No. 3,454,485, Japanese Patent Application Kokai (laid-open) No. 57-203945, Hitchman, M. L: Measurement of Dissolved Oxygen, published by John Wiley & Sons, Inc. (1978), Chapter 5, etc. Measurement of dissolved oxygen and hydrogen peroxide concentrations is disclosed in Japanese patent application Kokai (laid-open) No. 58-34353, measurement of dissolved SO.sub.2 in U.S. Pat. No. 3,756,923, and measurement of dissolved hydrocyanic acid, phosgene, and hydrogen sulfide concentrations in U.S. Pat. No. 4,227,974.
Measurement of dissolved hydrogen concentration in metallic sodium is disclosed in U.S. Pat. Nos. 3,649,473 and 3,683,272 and membranes for measuring dissolved gases in U.S. Pat. No. 3,767,552 and U.K. Patent Application No. GB2073430A.
Measurement of dissolved hydrogen concentration in core water is disclosed by the present applicants in Japanese Patent Application No. 57-97685=U.S. Patent Applicaton Ser. No. 500,367 (now abandoned)=Canadian Patent Application No. 429637=EPC Patent Application No. 83105626.2.
The basic structure of a membrane-type oxygen electrode so far used as a detector for quantitative determination of dissolved oxygen concentration of sample water at room temperature will be described below, referring to FIG. 1, which is a schematic, cross-sectional vertical view of a basic structure of prior art membrane-type dissolved oxygen meter according to the said Hitchman reference.
Oxygen, as dissolved in sample water 3, is continuously supplied to a measuring apparatus 100 from a sample water inlet 1 and discharged therefrom through a sample outlet 2, permeates a membrane 4 into an electrolyte 11, and the oxygen is reduced to OH.sup.- at a working electrode 10 according to the following equation (1) to generate a current between the working electrode 10 and a counter-electrode 5. EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- ( 1)
The generated current is measured by an ammeter 9. The working electrode 10 is kept at a desired potential by a potentiometer 7 and a DC current source 8. Basic change in current through a change in the working electrode potential is shown in FIG. 2, which is a characteristic diagram showing changes in output current of the dissolved oxygen meter shown in FIG. 1 through a change in the working electrode potential.
In FIG. 2, point A shows an equilibrium potential for oxidation-reduction reaction of oxygen, where no oxygen oxidation or reduction reaction takes place. When the working electrode potential is changed from point A toward more negative direction (toward right side in FIG. 2), the reduction reaction according to equation (1) proceeds to generate reduction current. The electron transfer rate at the electrode surface is a rate-determining factor for the current around the equilibrium potential, and the current is increased with changes in the working electrode potential toward more negative direction, and finally reaches a plateau showing a constant maximum value I.sub.l independent from the potential, where the oxygen permeation rate through the membrane is a rate-determining factor, and the current at the plateau is called "limiting current", which is proportional to an oxygen concentration of sample water. By selecting the potential at any point in this potential region producing the limiting current plateau and measuring a current while keeping the working electrode potential at that potential, dissolved oxygen can be quantitatively determined.
When the potential is made further more negative, the current starts to increase again (point B), because H.sup.+ reduction proceeds with the oxygen reduction according to the following equation (2). EQU 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 ( 2)
When there is hydrogen together with oxygen in core water, hydrogen also permeates through the membrane and is oxidized according to the following equation (3) to produce an oxidation current. EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 3)
It is known that the thus produced hydrogen oxidation current inteferes with the oxygen reduction current to reduce the output current by the oxygen reduction, as shown in the dotted line in FIG. 2, resulting in a measurement disturbance. To avoid such disturbance, an oxygen reduction current is measured in the prior art by keeping the working electrode potential around the equilibrium potential of H.sub.2 oxidation-reduction reaction as shown by point B in FIG. 2, whereby only a dissolved oxygen concentration is quantitatively determined.
The prior art has thus aimed at eliminating the hydrogen oxidation current as a disturbing current for the measurement of dissolved oxygen, and has not aimed at simultaneous measurement of dissolved hydrogen by utilizing the hydrogen oxidation current at all.